Offer and acceptance is like playing a game with your friend. Imagine your friend has a toy that you want to play with, and you ask if you can borrow it. If your friend says "yes", you have an offer and he has accepted your request.
An offer is when somebody suggests doing something or giving something to someone else. It's like asking someone if they want to play a game, or offering to trade snacks at lunch. When you make an offer, you are saying "I want to do something with you, and this is what I am suggesting."
Acceptance is when the other person agrees to the offer. For example, if you ask your friend if you can play with their toy and they say "yes", that means they have accepted your offer.
In order for an offer and acceptance to be complete, both parties need to understand what they are agreeing to. That means that if your friend doesn't understand what you mean when you ask to borrow their toy, then they haven't really accepted your offer.
So, offer and acceptance is about people agreeing to do things with each other. It's like making a plan, and everyone has to agree on the plan for it to work.